1. Field of the Invention
The present invention relates to a surface acoustic wave device employed as a high-frequency filter or the like, a surface acoustic wave apparatus, and communications equipment.
2. Description of Related Art
In recent years, surface acoustic wave filters have been employed for various types of communications equipment.
As the frequency and the function of the communications equipment have increased, a request to increase the out-of-band attenuation of the surface acoustic wave filter has progressively increased.
A schematic sectional view of a face-down mounting structure of a conventional surface acoustic wave device is illustrated in FIG. 13.
In FIG. 13, reference numeral 51 denotes a piezoelectric substrate, reference numeral 52 denotes a ground pad, reference numeral 53 denotes an IDT (Inter Digital Transducer) electrode (referred to as an IDT electrode) in a comb shape formed on the piezoelectric substrate 51, reference numeral 54 denotes a conduction pattern formed on a package (mounting substrate) 57, and reference numeral 55 denotes a connecting bump, and reference numeral 59 denotes a conductor layer formed on a reverse surface (a surface opposite to a surface on which the IDT electrode is formed) of the piezoelectric substrate 51.
In the configuration shown in FIG. 13, the ground pad 52 and the IDT electrode 53 are formed of an Al—Cu film, for example, and the conduction pattern 54 and the ground pad 52 are electrically connected to each other by the bump 55 composed of Au, for example. Further, a cover 56 is further subjected to seam welding or the like through a joint layer 58, thereby sealing the package 57 to maintain the internal airtightness of the surface acoustic wave device accommodating a surface acoustic wave device.
The main cause of degradation of an out-of-band attenuation in the conventional surface acoustic wave apparatus having such a face-down structure is, for example, input-output electromagnetic coupling due to an increase in electrical resistance of an electrode such as the ground pad 52, the IDT electrode 53, the conductor pattern 54 formed on the package 57, etc. in the surface acoustic wave device, a parasitic inductance, or a stray capacitance.
The input-output electromagnetic coupling due to the stray capacitance will be particularly described.
The surface acoustic wave device is a device using a comb-shaped IDT electrode produced on a piezoelectric substrate. Generally, a piezoelectric substance exhibits pyroelectric properties due to a rapid temperature change. When a device having an IDT electrode on a piezoelectric substrate is passed through a step in which there is a rapid temperature change while being manufactured, sparks are generated between electrode sections in the IDT electrode, thereby destroying the device. In order that charges may be stored in the piezoelectric substrate as little as possible, therefore, a conductor layer 59 is generally formed throughout the reverse surface of the piezoelectric substrate.
Although the conductor layer 59 is effective in preventing pyroelectric destruction during the element manufacturing process, however, the inventors found that a capacity coupling occurs between the conductor layer 59 and the input/output electrodes of the IDT electrode 53, which deteriorates the out-of-band attenuation.
In particular, a branching filter (a duplexer) for isolating a signal in a transmission-side frequency band (e.g., a low frequency-side frequency band) and a signal in a receiving-side frequency band (e.g., a high frequency-side frequency band) in the surface acoustic wave apparatus will be described in detail.
The branching filter is referred to as a surface acoustic wave duplexer (hereinafter abbreviated as an SAW-DPX).
In the SAW-DPX, a filter in the transmission-side frequency band (hereinafter referred to as a transmission-side filter) and a filter in the receiving-side frequency band (hereinafter referred to as a receiving-side filter) are formed on the same surface of the same piezoelectric substrate to achieve miniaturization.
When the transmission-side filter and the receiving-side filter are actually formed on the same piezoelectric substrate, however, isolation characteristics between both filters cannot satisfy requirement specifications in a communications terminal.
The isolation characteristics mean the level of a signal leakage from one filter to the other filter. Such a signal leakage must be miniaturized.
Particularly in the branching filter, when a high-power transmission signal amplified on the transmission side leaks from the transmission-side filter to the receiving-side filter to leak to the receiving side, an originally low-power receiving signal cannot be received.
In specifications of the isolation characteristics required for the branching filter, therefore, it is required that a signal leakage is significantly minimized. This requirement is significantly stricter than the requirement of specifications for a Dual-SAW filter employed between stages.
It is considered that one cause of degradation of the isolation characteristics between the filters is leakage of an elastic wave. Particularly in the SAW-DPX, it is considered that an elastic wave excited in an IDT electrode forming the transmission-side filter cannot be sufficiently trapped in the IDT electrode, and the elastic wave that has leaked from the IDT electrode in the transmission-side filter propagates on the surface of the piezoelectric substrate and is received by an IDT electrode forming the receiving-side filter so that the signal leaks from the transmission-side filter to the receiving-side filter, thereby degrading the isolation characteristics (Akinori Miyamoto, Shin-ichi Wakana, and Akio Ito, Fujitsu Laboratories Limited, “Novel optical observation technique for shear horizontal wave in SAW resonators on 42° YX-cut lithium tantalate” 2002 IEEE ULTRASONICS SYMPOSIUM-89).
Specifically, a propagation path of a surface acoustic wave from the IDT electrode in the transmission-side filter and a propagation path of a surface acoustic wave from the IDT electrode in the receiving-side filter are arranged so as to be overlapped with each other on the same straight line. Therefore, it is considered that the surface acoustic wave leaks from the IDT electrode in the transmission-side filter to the IDT electrode in the receiving-side filter, thereby degrading the isolation characteristics.
Therefore, an attempt has been made to improve the isolation characteristics by respectively separating the transmission-side filter and the receiving-side filter that have been formed on the same piezoelectric substrate and forming the filters on separate piezoelectric substrates to cut off the propagation of the leakage of the surface acoustic wave.
In such an attempt, the isolation characteristics are improved. However, the transmission-side filter and the receiving-side filter that have been originally integrally formed are separated and formed on the separate piezoelectric substrates. In a case where the transmission-side filter and the receiving-side filter are mounted on a mounting substrate, therefore, the area of a region serving as a branching filter is larger than that in a case where the transmission-side filter and the receiving-side filter are integrally formed on the same piezoelectric substrate. Therefore, it is impossible to fulfill a miniaturization requirement.
Therefore, it is also considered that the respective IDT electrodes in the transmission-side filter and the receiving-side filter are arranged such that the propagation paths of the surface acoustic wave from both IDT electrodes are parallel to each other, for example, so as not to be overlapped with each other. It should be possible to provide a small-sized SAW-DPX having improved isolation characteristics while forming the transmission-side filter and the receiving-side filter on the same piezoelectric substrate to achieve miniaturization without separating the filters on the separate piezoelectric substrates.
When the inventors of the present invention conducted detailed experiments, however, the isolation characteristics was not improved. This means that the degradation of the isolation characteristics does not result only from the leakage of the surface acoustic wave.
The inventors of the present invention found that the above-mentioned reverse surface conductor layer 59 is effective in preventing pyroelectric destruction during the device manufacturing process but is harmful for the isolation characteristics of the surface acoustic wave device.
If the reverse surface conductor layer and the ground electrode in the mounting substrate are conducted by wiring, therefore, capacitive coupling between the input and output electrode sections in each of the filters can be reduced to some extent. However, this measure does not sufficiently improve the isolation characteristics.
Furthermore, when the IDT electrode formation surface of the piezoelectric substrate and the main surface of the mounting substrate are opposed to each other to perform mounting (flip-chip mounting) with a vibration space ensured therebetween, it is advantageous in miniaturization. However, the reverse surface conductor layer on the reverse surface of the piezoelectric substrate is spaced apart from the main surface of the mounting substrate that can be at a ground potential. Therefore, extra steps are required to ground a portion from the reverse surface conductor layer to the ground electrode on the main surface of the mounting substrate, thereby increasing manufacturing costs are high.
An object of the present invention is to provide a surface acoustic wave device capable of improving the out-of-band attenuation of a filter and being superior in reliability, and communications equipment using the same.
Another object of the present invention is to provide a surface acoustic wave device, particularly, in which a transmission-side filter and a receiving-side filter are formed on the same piezoelectric substrate, being small in size and having superior out-of-band attenuation characteristics, and communications equipment using the same.